[mr48chev]

What are you using for power steering fluid? Automatic trans fluid while it functions doesn't work well in GM power steering. At least the older designs of Saginaw box and pump. If you have used that the best thing I can suggest is going to a GM dealer's parts counter and getting Genuine GM power steering fluid. When I worked in Pontiac dealer shops years ago I did all the power steering work and cured more than one makes noise power steering unit by flushing out the trans fluid and putting in the correct power steering fluid.

Front wheels off the ground/floor is a must when working the air out. Truth is I have never had one out of all that I had apart that I had trouble getting the air out of the system. That includes those that I had the box torn apart to the last recirculating ball.

Since the pump came off a donor rig it might be worth it to pull it off and check it out. Unless it sat for a long time without fluid in it or got hit hard on the end of the shaft there shouldn't be much problem. Being run dry is probably the hardest thing on the as the side plates get galled and don't seal well. I can write out step by step instructions for taking one apart and putting it back together if you need them. It isn't hard in the least but can be a bit tedious.